The present invention is directed to reinforced hollow tubes and their methods of manufacture and use. A specific application of the present invention is for arterial and venous cardiopulmonary bypass cannula. The present invention is particularly useful as the arterial return cannula or the venous withdrawal cannula for the cardiopulmonary bypass (CPB) system described in U.S. Pat. No. 5,584,803, the subject matter of which is incorporated herein by reference. The CPB system has an arterial cannula which is used to return oxygenated blood to the patient""s vascular system, and a venous cannula which is used to withdraw venous blood from the patient""s vascular system. An aortic occlusion catheter passes through the arterial cannula. The aortic occlusion catheter is used to block blood flow through the ascending aorta and deliver cardioplegic fluid to arrest the heart for performing surgery on the heart and great vessels. The aortic occlusion catheter is inserted through the same lumen in the arterial cannula which is used to return arterial blood to the patient so that the arterial blood essentially passes in the annular space between the aortic occlusion catheter and the arterial return cannula.
An advantage of the CPB system described above is that only one opening in the patient""s arterial system is required for both delivery of cardioplegic fluid and return of arterial blood. In order to achieve optimum blood and cardioplegic fluid flow rates, the wall of the arterial cannula must be minimized while retaining enough structural integrity to prevent kinking and/or cracking. The present invention is particularly useful in providing a thin walled cannula which may be used as an arterial return cannula for the system described above.
A known method of making a reinforced cannula is to dip a mandrel in a polymer solution and wrap a metal wire over the polymer. The mandrel is then dipped again to encase the metal wire between two layers of polymer.
Another known method of making a reinforced cannula is to extrude a polymer tubing, wrap a metal wire around the polymer tubing, and extrude another polymer layer over the metal wire.
A problem with the known methods of manufacturing a reinforced cannula is that the spacing between adjacent wires must be relatively large to ensure that the polymer flows between adjacent coils so that the two polymer layers bond together to form an integrated structure. Unfortunately, the relatively large spacing requires a relatively thick polymer layer to provide the necessary strength since the wire has a large pitch. The relatively thick polymer layer is also required to ensure that a sufficient amount of polymer is provided to fill the relatively large space. The resulting cannula therefore has a relatively thick wall.
Accordingly, there is a need in the art for an improved method of manufacturing reinforced tubing and, in particular, cannulae for venous withdrawal and arterial return of blood for use with a cardiopulmonary bypass system.
The present invention provides a cannula with at least a portion of its length reinforced, as well as a method of manufacturing the cannula.
According to one aspect of the invention, an elongate member, such as a steel or polymer wire, is coated with a coating, preferably a polymer, thereby forming a coated elongate member. A preferred method of coating the material is to coextrude the material over the elongate member. The coated elongate member is then wound helically around a mandrel and heated so that the coating on adjacent parts of the elongate member bond together. The coated elongate member is then mounted to a cannula body.
In one preferred embodiment, the coated elongate member is formed so that opposing sides of the coated elongate member engage one another when the coated elongate member is wrapped around the mandrel. A preferred cross-sectional shape is substantially square. An advantage of the present invention is that the coating does not need to flow between adjacent portions of the helically-wound member since the coated elongate members are configured to have sides which engage one another. In another aspect of the invention, the coated elongate member is compressed after being wound around the mandrel. The coated elongate member is preferably compressed with a heat shrink tube placed over the coated elongate member before heating. The shrink tube compresses the polymer to further ensure bonding between adjacent portions of the coated elongate member.
In a more specific preferred embodiment, a layer is positioned over and/or below the coated elongate member. The layer is preferably positioned over the coated elongate member and is applied as a tube of material having a larger inner diameter than the largest outer diameter of the coated elongate member. The tube is expanded, preferably by inflating the tube, and the coated elongate member is positioned inside the tube. The tube is then deflated so that it contracts around the coated elongate member. The tube and coated elongate member are then heated to fuse the elongate member and tube together to form an integrated structure. Although it is preferred to apply the layer as a tube, the layer may also be applied by dipping the coated elongated member in a suitable solution.
An advantage of a cannula constructed according to this aspect of the invention is that the cannula has a thin-walled construction while providing a lumen having a relatively large inner diameter. The lumen is particularly suited to receive an aortic occlusion catheter while still providing enough annular area between the catheter and lumen wall for return of arterial blood to sustain full CPB.
According to another aspect of the invention, a cannula is constructed so as to include reinforced sections and nonreinforced sections and a lumen passing through the sections. The respective sections are preferably alternately disposed along a portion or all of the length of the cannula. For example, a distal portion of the cannula may include alternating reinforced and nonreinforced sections, while the remaining length of the cannula is reinforced. Alternatively, the proximal and distal portions of the cannula may include alternating reinforced and non-reinforced sections, while the cannula is reinforced between these portions. As another alternative, the entire length of the cannula may include alternating reinforced and nonreinforced sections.
The reinforced sections of the cannula comprise tubular sections which include a reinforcing member while the nonreinforced sections comprise tubular sections which are substantially free of the reinforcing member. One or more openings are formed in the nonreinforced sections and serve as entry or exit ports for fluid being withdrawn from or delivered to a patient""s vascular system. For example, the cannula may be used to withdraw venous blood from a patient""s vascular system which is delivered to and oxygenated by a cardiopulmonary bypass system. In addition, or alternatively, the cannula may be used to deliver oxygenated blood from the cardiopulmonary bypass system to the patient""s vascular system.
In one preferred embodiment, the reinforced sections of the cannula comprise a reinforcing member encased in a material, while the nonreinforced sections of the cannula comprise plain tubing sections. As an example, the reinforcing member may be elongate stainless steel wire, while the material encasing the wire is a suitable polymer.
According to this aspect of the invention, a method of making a cannula for withdrawing fluid from or delivering fluid to a patient""s vascular system is also provided. In a preferred embodiment, the method comprises steps of forming a reinforced tubular body comprising a reinforcing member, and separating the reinforced tubular body to form a plurality of separate reinforced tubular sections each of which comprises the reinforcing member. A tubular section which is substantially free of the reinforcing member is disposed between at least two of the reinforced tubular sections, and the sections are bonded together to form a cannula having separate reinforced sections and a lumen extending therethrough. At least one opening is formed in the cannula between the reinforced sections such that fluid can flow through the opening and the lumen of the cannula.
In yet another aspect of the invention, a cannula comprises a tubular body having one or more openings disposed at one or more locations which are proximal to a midpoint of the cannula, the openings passing through the cannula so as to communicate with a lumen extending through the cannula. The openings are configured to allow fluid to pass into or out of the cannula along a proximal portion thereof, as opposed to only a distal portion thereof, as is conventional. When the cannula is used as a venous withdrawal or arterial return cannula for a CPB system, some of the blood being passed into or out of the cannula enters the proximal openings. As a result, it is not necessary to deliver all of the blood between the CPB system and the distal portion or end of the cannula, thereby allowing reduced pressure differentials to drive fluid flow, as compared with conventional cannulae. In one preferred embodiment, the cannula includes reinforced and nonreinforced sections, and the openings are formed in the nonreinforced sections.